This invention relates generally to a photoelastic measurement device and more particularly to such a device for measuring loads on a structural member.
In the past, a variety of strain guages, mechanical extensometers and hydraulic devices have been used for this purpose. However, these previous devices have had a number of disadvantages. Those of simple and economic design have not provided the requisite sensitivity and accuracy, while the more complex of these previous devices have been found to be prohibitively expensive. In addition, they are difficult to maintain in accurate adjustment as they are generally subject to physical effects such as changes in temperature and humidity, with automatic compensation being difficult.
Although in describing the present invention, reference is made to embodiments directed to the measurement of load on a crane, it is to be understood that the invention is not restricted to these embodiments, as the device may be used in a large variety of applications wherein a load measuring device is desired which provides an economical combination of simplicity, accuracy and reliability. In this particular field, previous load measurement devices located on the structure or on the cable of a crane have had the disadvantages that they are highly susceptible to damage, and subject to inaccuracies resulting from shock and vibration. In addition, devices located on the cable are subject to wear and inaccuracies occur due to changes in cable dimension with variations in load. Furthermore, these previous devices have not provided suitable automatic compensation for boom angle and ambient temperature variations. The better of these previous devices are very expensive, and even the most satisfactory of them require an extensive degree of maintenance to provide the necessary degree of accuracy and repeatability required.